This invention generally relates to wideband antennas, and in particular to antennas for transmitting and receiving ultrawideband (UWB) signals.
Techniques for UWB communication developed from radar and other military applications, and pioneering work was carried out by Dr G. F. Ross, as described in U.S. Pat. No. 3,728,632. Ultra-wideband communications systems employ very short pulses of electromagnetic radiation (impulses) with short rise and fall times, resulting in a spectrum with a very wide bandwidth. Some systems employ direct excitation of an antenna with such a pulse which then radiates with its characteristic impulse or step response (depending upon the excitation). Such systems are referred to as carrierless or “carrier free” since the resulting rf emission lacks any well-defined carrier frequency. However other UWB systems radiate one or a few cycles of a high frequency carrier and thus it is possible to define a meaningful centre frequency and/or phase despite the large signal bandwidth. The US Federal Communications Commission (FCC) defines UWB as a −10 dB bandwidth of at least 25% of a centre (or average) frequency or a bandwidth of at least 1.5 GHz; the US DARPA definition is similar but refers to a −20 dB bandwidth. Such formal definitions are useful and clearly differentiates UWB systems from conventional narrow and wideband systems but the techniques described in this specification are not limited to systems falling within this precise definition and may be employed with similar systems employing very short pulses of electromagnetic radiation.
UWB communications systems have a number of advantages over conventional systems. Broadly speaking, the very large bandwidth facilitates very high data rate communications and since pulses of radiation are employed the average transmit power (and also power consumption) may be kept low even though the power in each pulse may be relatively large. Also, since the power in each pulse is spread over a large bandwidth the power per unit frequency may be very low indeed, allowing UWB systems to coexist with other spectrum users and, in military applications, providing a low probability of intercept. The short pulses also make UWB communications systems relatively unsusceptible to multipath effects since multiple reflections can in general be resolved. The use of short pulses also facilitates high resolution position determination and measurement in both radar and communication systems. Finally UWB systems lend themselves to a substantially all-digital implementation, with consequent cost savings and other advantages.
FIG. 1a shows an example of a UWB transceiver 100 comprising a transmit/receive antenna 102 coupled, via a transmit/receive switch 104, to a UWB receiver 106 and UWB transmitter 108. In alternative arrangements separate transmit and receive antennas may be provided.
The UWB transmitter 108 may comprise an impulse generator modulated by a base band transmit data input and, optionally, an antenna driver (depending upon the desired output power). One of a number of modulation techniques may be employed, for example on-off keying (transmitting or not transmitting a pulse), pulse amplitude modulation, or pulse position modulation. A typical transmitted pulse is shown in FIG. 1b and has a duration of less than 1 ns and a bandwidth of the order of gigahertz.
FIG. 1c shows an example of a carrier-based UWB transmitter 120. This form of transmitter allows the UWB transmission centre frequency and bandwidth to be controlled and, because it is carrier-based, allows the use of frequency and phase as well as amplitude and position modulation. Thus, for example, QAM (quadrature amplitude modulation) or M-ary PSK (phase shift keying) may be employed.
Referring to FIG. 1c, an oscillator 124 generates a high frequency carrier which is gated by a mixer 126 which, in effect, acts as a high speed switch. A second input to the mixer is provided by an impulse generator 128, filtered by an (optional) bandpass filter 130. The amplitude of the filtered impulse determines the time for which the mixer diodes are forward biased and hence the effective pulse width and bandwidth of the UWB signal at the output of the mixer. The bandwidth of the UWB signal is similarly also determined by the bandwidth of filter 130. The centre frequency and instantaneous phase of the UWB signal is determined by oscillator 124, and may be modulated by a data input 132. An example of a transmitter with a centre frequency of 1.5 GHz and a bandwidth of 400 MHz is described in U.S. Pat. No. 6,026,125. Pulse to pulse coherency can be achieved by phase locking the impulse generator to the oscillator.
The output of mixer 126 is processed by a bandpass filter 134 to reject out-of-band frequencies and undesirable mixer products, optionally attenuated by a digitally controlled rf attenuator 136 to allow additional amplitude modulation, and then passed to a wideband power amplifier 138 such as a MMIC (monolithic microwave integrated circuit), and transmit antenna 140. The power amplifier may be gated on and off in synchrony with the impulses from generator 128, as described in U.S. Pat. No. '125, to reduce power consumption.
FIG. 1d shows a block diagram of a UWB receiver 150. An incoming UWB signal is received by an antenna 102 and provided to an analog front end block 154 which comprises a low noise amplifier (LNA) and filter 156 and an analog-to-digital converter 158. A set of counters or registers 160 is also provided to capture and record statistics relating to the received UWB input signal. The analog front end 154 is primarily responsible for converting the received UWB signal into digital form.
The digitised UWB signal output from front end 154 is provided to a demodulation block 162 comprising a correlator bank 164 and a detector 166. The digitised input signal is correlated with a reference signal from a reference signal memory 168 which discriminates against noise and the output of the correlator is then fed to the detector which determines the n (where n is a positive integer) most probable locations and phase values for a received pulse.
The output of the demodulation block 162 is provided to a conventional forward error correction (FEC) block 170. In one implementation of the receiver FEC block 170 comprises a trellis or Viterbi state decoder 172 followed by a (de) interleaver 174, a Reed Solomon decoder 176 and (de) scrambler 178. In other implementations other codings/decoding schemes such as turbo coding may be employed.
The output of FEC block is then passed to a data synchronisation unit 180 comprising a cyclic redundancy check (CRC) block 182 and de-framer 184. The data synchronisation unit 180 locks onto and tracks framing within the received data separating MAC (Media Access Control) control information from the application data stream(s) providing a data output to a subsequent MAC block (not shown).
A control processor 186 comprising a CPU (Central Processing Unit) with program code and data storage memory is used to control the receiver. The primary task of the control processor 186 is to maintain the reference signal that is fed to the correlator to track changes in the received signal due to environmental changes (such as the initial determination of the reference waveform, control over gain in the LNA block 156, and on-going adjustments in the reference waveform to compensate for external changes in the environment).
There are demanding requirements on antennas suitable for UWB communications and other UWB applications such as UWB radar. The most obvious requirement is for an antenna with a very wide bandwidth. Conventionally an antenna is considered broadband if the ratio of maximum to minimum frequency of operation of the antenna is only 1.2:1, where the maximum and minimum operating frequencies are defined by, for example, the 3 dB received signal power points (at which the received signal power falls to half its centre or maximum in-band value). Ultrawideband systems, however, generally require ratios of 2:1 or 3:1. However for many applications a broadband frequency response is not enough and a good phase response across the band is also required. This can be seen by considering the effects of dispersion in the time domain in the above described receiver. In order to properly capture a received UWB signal components of a pulse should have a maximum displacement in time from one another which is much less than the period of the highest frequency component of the signal present at a significant level. For example where a UWB signal has an upper roll-off frequency of, say, 10 GHz, corresponding to a period of 100 ps the time (or phase) dispersion should preferably be significantly less than 100 ps. As the skilled person will appreciate low phase dispersion translates to low frequency dispersion.
One conventional broadband antenna is the log periodic array, which comprises a string of dipole antennas fed alternately by a common transmission line. The dipole antennas are of different lengths in order to provide a set of overlapping frequency responses. However because the dipole elements are spaced apart on the antenna, different frequency components reach the antenna at different times and thus the effective position of the antenna moves with frequency, giving rise to time/phase dispersion.
Another wideband antenna is the biconical antenna, the shape of which is substantially frequency independent. An example of an ultrawideband biconical antenna is described in U.S. Pat. No. 5,923,299. Biconical antennas can, however, have difficulties providing a sufficiently flat, wideband response and the biconical shape is relatively bulky, complex and expensive to manufacture.
Tapered slot or Vivaldi antennas have a theoretically infinite bandwidth but in practice there are difficulties providing a suitable feed to such an antenna. The antennas can also be relatively costly to manufacture. An example of a UWB antipodal tapered slot antenna is described in WO02/089253.
A cross-polarised UWB antenna system comprising a magnetic dipole slot antenna and an ultrawideband dipole antenna is described in, inter alia, WO99/13531, U.S. Pat. No. 6,621,462, and US2002/0154064. Again, however, this is a relatively complex configuration and the dipole shape appears to be based upon the principle of spreading the resonance of the antenna by, in effect, reducing the Q, but nonetheless the design would appear to exhibit significant potential for undesired resonances.
An elliptical planar dipole UWB antenna is described in US 2003/0090436 but the elliptical shape is non-optimal and the antenna apparently works by establishing current flows around the periphery of the antenna.
One commercially available broadband antenna which can be utilised for UWB communications is the SMT-3TO10M from SkyCross Corp., Florida USA, which comprises a form of folded dipole.
Other background prior art can be found in U.S. Pat. No. 5,973,653, EP1 324 423A, US 2003/011525, US 2002/126051, USH1773H, WO98/04016, U.S. Pat. No. 5,351,063, EP0 618 641 A, and in ‘Antennas’ by John D Kraus and Ronald J Marhefka, McGraw Hill 2002 3/e (for example at page 782, which describes a resistance-loaded bow-tie antenna for ground penetrating radar). Helical antennas are sometimes employed to provide circular polarisation. Circular patch antennas are known but these are relatively narrowband devices (their bandwidth does not approach that desirable in a UWB system) comprising a circular area of copper parallel to a ground plane.